Commercial freezers typically rely on the transfer of heat from a food product that is to be chilled or frozen by using a fan or blower. Typically, the fan or blower is situated near a conveyer belt upon which the food product is being carried. The food product entering the freezer has a boundary layer of air surrounding it which insulates the food product from the surrounding atmosphere. Traditional freezers have employed blowers that generate currents of cooling vapor in many directions. However, a significant portion of the cooling vapor does not contact the food product in a perpendicular direction. Under these conditions, the cooling vapor which does contact the food product often does not possess sufficient energy to substantially reduce the thickness of the boundary layer around the surface of the food product. Therefore, there is a need to generate directed jets of cooling vapor so as to disturb the boundary layer.
U.S. Pat. No. 4,479,776 to Smith discloses an apparatus using a plurality of vertical tubes to provide a unidirectional air flow toward the food product.
U.S. Pat. No. 4,626,661 to Henke discloses the use of a plurality of nozzles along the pathway of a food product for delivering discrete jets of unidirectional cooling air.
However, the use of tubes or nozzles to direct air in a cooling or freezing device has met with only limited success due to the build-up of condensation in the form of snow or ice in the tubes or nozzles. Such build up quickly reduces the efficacy of the cooling or freezing devices.
U.S. Pat. No. 5,487,908 to Appolonia et al. discloses a method and device for heating or cooling a food product on a moving substrate in which a continuous channel traversing at least a major portion of the width of the moving substrate converts multi-directional flow into unidirectional flow. However, such a device suffers from having such an increased rate of flow that the food products become entrained in the flow, and, consequently, controlled processing of the food item through the device becomes difficult.
Increasing the velocity of the stream of cooling vapor (or cryogen) which impinges the food item will increase the average heat transfer coefficient in a linear manner. At a certain point, however, unless the impingement stream of cooling vapor is carefully controlled, the velocity may also be sufficient to damage the food product, or to carry the food product off the conveyor, and into undesirable locations elsewhere in the freezer.
The total heat transfer rates are dependent on local heat transfer coefficients. That is, the amount of heat transferred from the food products to the cryogen is dependent on the rate of heat transfer locally between the cryogen and the food product. Local heat transfer rates can be changed by controlling the distance from the source of impingement stream to the food product, the velocity of the impingement stream, the turbulence in the impingement stream, and the efficiency of the flow of cryogen.
Therefore, a need remains for a device which can rapidly chill and/or freeze a food product with the heat transfer to a cryogen, such as CO2 or N2, while reducing the amount of cryogen needed by extracting the maximum cooling effect from a given amount of the cryogen. The device must also be capable of transporting food product from an inlet to an outlet without damaging the food product. Additionally, the device must be able to control the throughput of food items, and must be resistant to the freezing and plugging of internal components by snow and ice build-up.